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This article will explain in detail what are the advanced features of LINQ. Xiaobian thinks it is quite practical, so share it with you as a reference. I hope you can gain something after reading this article.

LINQ Advanced Features Dynamic Query

One scenario is that an application might provide a user interface that allows users to specify one or more predicates to filter data. In this case, the query details are not known at compile time, and dynamic queries can be useful.

In LINQ, Lambda expressions are the basis for many standard query operators, and the compiler creates lambda expressions to capture the calculations defined in the underlying query methods, such as Where, Select, Order By, Take While, and others. Expression catalog trees are used for structured queries against data sources that implement IQueryable. For example, the LINQ to SQL provider implements the IQueryable interface for querying relational data stores. C#and Visual Basic compilers compile queries against such data sources into code that generates an expression catalog tree at runtime. The query provider can then traverse the expression tree data structure and convert it into a query language appropriate for the data source.

The expression tree is used in LINQ to represent Lambda expressions assigned to variables of type Expression. It can also be used to create dynamic LINQ queries.

The System.Linq.Expressions namespace provides an API for manually generating expression catalog trees. The Expression class contains static factory methods that create a specific type of expression tree node, such as ParameterExpression (representing a named parameter expression) or MethodCallExpression (representing a method call). The root of the expression tree generated by the compiler is always in a node of type Expression, where TDelegate is any TDelegate with up to five input parameters; that is, its root node is a lambda expression.

The following examples describe how to use the expression tree to create dynamic LINQ queries.

LINQ Select Advanced Features

The following example shows how to use the expression tree to construct a dynamic query from the IQueryable data source, query the ContactName of each customer, and use the GetCommand method to obtain it to generate SQL statements.

//construct a query from an IQueryable data source

IQueryable custs = db.Customers;

//Create an expression tree to create a parameter

ParameterExpressionparam =Expression.Parameter(typeof(Customer),"c");

//Create an expression tree

c.ContactNameExpressionselector =Expression.Property(param,typeof(Customer).

GetProperty("ContactName"));

ExpressionExpressionpred =Expression.Lambda(selector, param);

//Build expression tree:Select(c=>c.ContactName) ExpressionExpressionr =Expression.Call

(typeof(Queryable),"Select",newType[] {typeof(Customer),typeof(string) },

Expression.Constant(custs), pred);

//Use expression trees to generate dynamic queries IQueryable query = db.Customers.AsQueryable()

.Provider.CreateQuery(expr);

//Get SQL statement using GetCommand method

System.Data.Common.DbCommandcmd = db.GetCommand(query);

Console.WriteLine(cmd.CommandText);

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